1. Field of the Invention
The present invention relates to disk drives, and in particular to a head-gimbal assembly which is mounted on an actuator arm by a novel swaging process.
2. Description of Related Art
Conventional disk drives for use in workstations, personal computers, and portable computers are required to provide a large amount of data storage within a minimum physical space. In general, Winchester type disk drives operate by transferring data between read/write transducing heads and one or more rotating magnetic storage disks. Positioning of the heads at the desired location over respective data tracks on the disk is accomplished by an actuator assembly coupled to control electronics. The electronics control rotation of the disk, positioning of the actuator assembly and the read/write functions of the heads.
Greater demands are being placed on disk drives by (1) the use of multi-user and/or multi-tasking operating systems, (2) workstations which provide an operating environment requiring the transfer of large amounts of data to and from a hard disk and/or large numbers of disk accesses to support large application programs or multiple users, (3) the present popularity of notebook and laptop computers, and (4) the continuing trend toward higher performance microprocessors. All such systems require a hard drive having high capacity storage capability, which minimizes destructive contact between the head and disk.
Since disk drives will most likely remain the primary memory device in computer applications, disk drives will require greater storage capacities, while at the same time becoming smaller, faster and less expensive. These changes will mean greater emphasis on lower flying heights for heads, higher data transmission speeds, and lower costs of disk drive assemblies. In order to accommodate these demands, the suspension arm of the head-gimbal assembly (HGA) must have low friction and stresses at its interface with the actuator arm in a disk drive. Suspension assemblies will become more precise and an important part of the essential high speed data transmission path. To this end, there is a need for an improved method of mounting a head-gimbal assembly on an actuator arm which eliminates formation of kinematic deflections due to the mounting process. Kinematic deflections cause the heads to fly at different heights thus effecting the precision and high performance of the suspension assemblies in the disk drive.
A HGA consists of a read/write head attached to a suspension assembly. A suspension assembly holds the read/write head in position while pressing it towards the disk surface with a precise force applied in a precisely determined location. The head flies above the disk at a height established by the equilibrium of the suspension force and the increasing force of the air stream under the head as the head nears the disk. Any twist (torque) imparted by the suspension to the head will affect the flying height, as will variations in the position of the applied force. It is therefore important for head and suspension designers to seek the optimal combination of force, location and twist which will produce the desired flying height, greatest stability and greatest immunity to disturbances.
An important determinant in establishing a stable head and suspension assembly is the swaging process. The swaging process is a method for mounting the HGA on the actuator arm of an actuator assembly. In a conventional swaging process as shown in FIGS. 1-3, a swage boss 2 on a baseplate 4 welded to an suspension arm (not shown) of the HGA is placed in a hole 6 of an actuator arm S. A steel ball 10 having a diameter which is slightly larger than swage boss hole 7 in the swage boss 2, is pressed through a baseplate hole 5 and swage boss hole 7. During the swaging process, the swage boss 2 and the baseplate 4 deform plastically to form a press fit at 12 with the actuator arm 8.
While conventional swaging processes are cost effective, these processes cause the actuator arm and the baseplate on the HGA to permanently deform which results in variations in the "z-height" and gram load. The z-height 14, as shown in FIG. 4, is the vertical distance between the disk surface 16 and the actuator arm 18 on which the baseplate of the HGA is mounted. The z-height variations resulting from conventional swaging processes adversely affect the uniformity of the flying height at respective head/disk interfaces. As multiple HGA's are assembled with other components into a head stack assembly, and as the industry transitions to smaller form factors, the variations in the z-height are likely to further exacerbate the problem of achieving a stable suspension assembly.
It is believed that the variation in the z-height occurs due to the application of a large force (usually 170-200 lbs.) applied in a direction perpendicular to the actuator arm during the swaging process. Due to the force in the swaging process, the spring characteristics of the suspensions change, thereby creating a change in the gram load and the z-height. The gram load is the measure of the force exerted by the suspension on the disk via the slider which supports the transducer. Furthermore, swaging is a dynamic process in which a situation of direct impact prevails and causes continuous distortion as the swaging ball moves from one swage boss to the another swage boss in a stack assembly. Direct impact is a collision between two bodies where relatively large forces result over a comparatively short interval of time. The bodies travel towards each other on a line joining their centers of mass. In the swaging process, the swaging ball initially directly impacts the first baseplate on an arm and upon exiting it, impacts the next baseplate at a high speed with a high kinetic energy. The high kinetic energy is absorbed in the deformation of the baseplate and the actuator arm. The situation repeats itself as the swaging ball impacts the second arm, third arm and so on. If both of these problems are eliminated, the integrity of the final head stack assembly would be greatly enhanced.
Apart from the high precision and equilibrium demands of mounting the HGA to the actuator arm, it is required that the actuator arm and suspension arm of the HGA are able to withstand a minimum torque of magnitude 20 oz.-in. at the interface between them. This is the torque necessary to disassemble the HGA from the actuator arm. The actuator arm and suspension arm must not experience permanent plastic deformation during the swaging process because there must be the capability to rework the assembly for troubleshooting and repair purposes. Plastic deformation implies a loss of "elasticity" of the material due to the absorption of kinetic energy during the swaging process. Plastic deformation of the actuator arm will produce a permanent change in the arm resulting in a reduction of the retention force exerted by the actuator arm on the baseplate. This creates an undesirable effect on the swaging of a reworked actuator arm. Therefore, it is imperative that the residual stress levels at the interface of the swage boss and actuator arm not exceed the yield limit of the actuator arm material.